Conventionally, this kind of contrast adjusting circuit is composed of a multiplying circuit 10 as is shown in FIG. 1, the multiplying circuit 10 being designed to obtain the output video data Y by multiplying the video data X by a variably set slope A.
When the input video data X and the output video data Y are represented by X (X>=0) and Y (Y>=0) of the X-Y coordinates, there holds a relationship, Y=A X, an expression representing a straight line A including the original (0, 0), between the input video data X and the output video data Y as shown in FIG. 2. That is, assuming that the variable ranges of the slope A to be from A1 to Am (A1=<A=<Am, m being an integer 2 or more), the contrast increases when the slope A is increased (e.g., Y=Am·X), whereas the contrast decreases when the slope A is decreased (e.g., Y=A1·X).
However, in the case of the prior art as is shown in FIG. 1 there occurs a problem such that, when the contrast is set to a high level by setting the slope A to a high level (e.g., A=Am), the luminance of the image corresponding to low-level input video data X (i.e., low input level) becomes too high causing the image to become whitish.
Another problem of the prior art has been that the optimal contrast adjustment cannot be made to the ever-changing conditions of the image.
The present invention is made in consideration of the problem of the prior art and is first intended to provide a contrast adjusting circuit capable of controlling the increase in low-level luminance at the time when the contrast is increased, thereby preventing the image from becoming whitish.
The second object of the present invention is to provide a contrast adjusting circuit capable of adjusting the contrast to an optimal level corresponding to the ever-changing conditions of the image.